This invention relates to spectroscopy and particularly to Fourier transform spectroscopy, in which a Michelson interferometer is generally used. The pattern traced out, as the length of one interferometer arm is scanned, is the Fourier transform of the wavelength spectrum.
This invention more specifically relates to a "dual beam" Fourier spectrometer. For the present discussion, the term "dual beam" will refer to an optical system incorporating a Michelson interferometer and having two distinct optical paths in either the input or output of the interferometer (or both). In a dual beam Fourier spectrometer, these paths are used to simultaneously obtain (a) data from a material sample under study and (b) data for reference purposes from a sample-free region. This use of the term dual beam should be distinguished from the use of the same term to refer to the fact that the Michelson interferometer is itself a "dual beam" system, i.e., an interferometer in which light is separated by the beamsplitter into two beams and then recombined.
"Dual beam" spectroscopy is the subject of Chapter 7 of the book "Chemical Infrared Fourier Transform Spectroscopy" by Griffiths, published by John Wiley & Sons.
The importance of dual beam Fourier spectroscopy (FS) stems from the fact that the interferogram corresponding to a broad radiation spectrum will have a very high peak value (central maximum) when the path lengths of the two interferometer arms are equal (see Griffiths, FIG. 1.11). In order to electronically perform the Fourier transformation required to obtain the spectrum, it is first necessary to digitize the interferogram, and the cost of Analog-to-Digital (A/D) converters rises rapidly with increased resolution. As discussed by Griffiths (Chapter 2, Sec. II), the resolution required of the input A/D converter will normally be much greater than that of the resulting spectrum.
The dual beam approach can be applied when the Fourier spectrometer is being used to study the spectral characteristics of materials in either transmission or reflection. For the discussion which follows, we will assume the transmission case.
Griffiths describes several systems that have been proposed for dual beam Fourier spectrometry. The simplest example is the system of Burroughs and Chamberlain (B and C), FIG. 7.7. This system uses a conventional interferometer with the exception that retro-reflectors are used as interferometer mirrors. This allows the input and output beams to be physically displaced from each other within the interferometer. As a result, the output beam which emerges on the same side of the beamsplitter as the input beam can now be detected. The AC portion of the interferogram observed in this beam has the opposite polarity from that observed in the beam emergent from the opposite side of the beamsplitter. If both beams are detected and the resultant electric signals added, the two interferograms will cancel, giving no AC signal as long as the two optical paths and detector responses are identical.
In using a dual beam Fourier Spectrometer of the type just described, a sample to be studied is introduced into the path of one of the beams (the sample beam). The absorption characteristics of the sample alter the characteristics of the sample beam interferogram so that it is no longer exactly the inverse of the reference beam interferogram. As a result, the summed output signal will exhibit a net interferogram which is due only to the absorption properties of the sample. This interferogram will generally have a much smaller central maximum than the single beam interferogram, but can still be processed to yield the absorption spectrum of the sample.
According to Griffiths, the system described above is probably the preferred approach to dual beam Fourier spectrometry. However, it has the disadvantage of requiring large area retro-reflectors and a beamsplitter with high optical quality over the full aperture. And, needless-to-say, precise control of the motion of a large retro-reflector is difficult. In addition, the fact that the two beams emerge from opposite sides of the beamsplitter means that they will be affected in different ways by the properties of the beamsplitter. Of particular significance is the effect of the second (low reflectance) surface of the beamsplitter. The resultant imperfect matching of the two beams can be quite significant. This problem is recognized by Griffiths (see pages 177 and 183) who states that it may, in part, be caused by oxides of germanium on the upper surface of the beamsplitter (page 177).
In addition to the design described above, Griffiths discusses several other dual beam spectrometers, some of which have the separated beams in the source portion of the system, rather than the detector portion. All of these systems share the common feature that the two beams either emerge from or enter opposite sides of the beamsplitter. They thus share the beamsplitter asymmetry problem mentioned above.